virtio-scsi: restart DMA after iothread

When the 'cont' command resumes guest execution the vm change state
handlers are invoked.  Unfortunately there is no explicit ordering
between classic qemu_add_vm_change_state_handler() callbacks.  When two
layers of code both use vm change state handlers, we don't control which
handler runs first.

virtio-scsi with iothreads hits a deadlock when a failed SCSI command is
restarted and completes before the iothread is re-initialized.

This patch uses the new qdev_add_vm_change_state_handler() API to
guarantee that virtio-scsi's virtio change state handler executes before
the SCSI bus children.  This way DMA is restarted after the iothread has
re-initialized.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>